Check sequence-level gating in render_xblock (TNL-7636).

There is certain gating logic around pre-reqs, timed exams, etc.
that happen at the SequenceModule level, and should be respected
when rendering descendant XBlocks (like individual problems) that
are in that Sequence. Rather than do a risky refactoring, I'm
keeping that logic where it is and having the render_xblock view
climb up through the ancestor list to call the SequenceModule for
that gating information.

We do _not_ check all descendants (so cousin leaf nodes in the
sequence) for cotent-type-based restrictions because sequences can
become very large (esp. when content libraries are used), and there
is a performance overhead.

If the enclosing sequence is gated in some way, we redirect to the
render_xblock view for that sequence, where hopefully some useful
messaging will be available. This is a stopgap. That redirect
should never happen because we should never be calling the leaf
XBlock for a sequence that is restricted in the MFE. But if somehow
we get there anyway, either by bug or by intrepid user fiddling,
it's better to redirect somewhere that an error _might_ be surfaced
rather than just failing.

This will actually be a little overzealous and lock things down
that should be made visible later. If there's a timed exam and the
exam is completed, it should be the case that content is visible
(just read-only). This commit will block the content before the exam
starts (this is right), open the content while the exam is live
(this is right), but make the content unavailable after the exam
period has finished (this is wrong).

But I am going to go forward with this even knowing it's wrong
because:

1. The render_xblock endpoint should never currently be used in
   timed exams in an intentional way. Neither the mobile experience
   nor the courseware MFE support it.
2. This fix will address security concerns for creative access
   patterns, even if it goes too far.
3. We're going to need to do a lot of work to address both pluggable
   access permissions handling and special exams in the courseware
   MFE, and a better implementation can be done then.
4. I've had multiple failed attempts to get this to work without
   breaking things on and off over the course of weeks, and this
   is a relatively low risk way of doing it that doesn't involve
   a major refactoring (though the bill for that will come due
   when we bring timed exams to the MFE).